CONTROL CIRCUIT FOR VEHICLE RETARDATION

摘要

A control circuit for vehicle retardation has a hydraulic retarder (26) and a slipping friction brake (14) operating in concert to retard the forward motion of the vehicle. The hydraulic retarder is an output driven device which is operable to absorb increasing energy as the vehicle speed increases. The friction brake is normally a ratio control reaction brake for the lowest forward drive ratio of the vehicle transmission. The friction brake has an additional apply piston (84,86) which is operable to slippingly engage the friction brake whenever the hydraulic retarder is operable and the vehicle transmission is not in the lowest forward ratio. The control circuit has a control valve (52) which is operable to control the engagement pressure of the additional piston in a manner such that the friction braking effort is higher at low vehicle speeds. This compensates for the lower effectiveness of the output driven hydraulic retarder. The control circuit is also effective to control the operation of the hydraulic retarder, the main piston of the reaction brake, cooling flow to the friction brake and cooling flow from the hydraulic retarder.